1. Field of the Invention
One disclosed aspect of the embodiments relates to a method for driving an image pickup apparatus having an analog-to-digital (AD) conversion unit which converts an analog signal into a digital signal, the image pickup apparatus, and an image pickup system.
2. Description of the Related Art
Image pickup apparatuses are known which have an AD conversion unit which converts an analog signal into a digital signal. As an exemplary AD conversion unit, as described in Japanese Patent Laid-Open No. 2011-114785, an image pickup apparatus is known which has an AD conversion unit which performs two stages of AD conversion. In the image pickup apparatus described in Japanese Patent Laid-Open No. 2011-114785, the AD conversion unit is operated as a successive approximation AD conversion circuit to specify a subrange. Then, the AD conversion unit is operated as a ramp AD conversion circuit using a ramp signal, to perform AD conversion on a signal in the specified subrange. Japanese Patent Laid-Open No. 2011-114785 describes a technique in which, in order to reduce occurrence of discontinuities in input-output characteristics between adjacent subranges, the AD conversion unit is used as a successive approximation AD conversion circuit to convert a test signal into a first digital signal. The value of the test signal is adjusted so that the value of the first digital signal becomes equal to a predetermined value. Then, the AD conversion unit is used as a ramp AD conversion circuit to convert the test signal whose value has been adjusted, into a second digital signal. The clock frequency for driving a circuit which generates a ramp signal is adjusted so that the value of the second digital signal becomes equal to a predetermined value. Accordingly, the change width of the potential of the ramp signal per unit time is adjusted. That is, Japanese Patent Laid-Open No. 2011-114785 describes correction of a signal used in AD conversion of an analog signal.
Japanese Patent Laid-Open No. 2011-250039 describes an image pickup apparatus having an AD conversion unit which functions as a flash AD conversion circuit which determines the higher-order bits and as a ramp AD conversion circuit which determines the lower-order bits. In the technique described in Japanese Patent Laid-Open No. 2011-250039, a potential range which is input into the flash AD conversion circuit is divided into four sections. A correction coefficient for eliminating the difference between data obtained by the ramp AD conversion circuit and ideal data which is to be obtained is calculated for each of the four sections obtained through division. The correction coefficients are used to correct a digital signal which is output by the AD conversion circuit. That is, Japanese Patent Laid-Open No. 2011-250039 describes correction of a digital signal generated through AD conversion.